1. Field of the Invention
The invention relates to an apparatus according to the preamble of claim 1 for carrying out chemical and physical processes comprised of two separate unit processes using a particulate matter medium.
2. Background of the Related Art
Generally, such an apparatus includes a reactor for performing the first unit process, a regenerator for performing the second unit process and a communicating channel arranged between said reactor and said regenerator for the transfer of said particulate matter medium from the reactor to the regenerator as well as a return channel thereinbetween for returning regenerated particulate matter medium from the regenerator back to the reactor for reuse.
A great number of processes in the chemical and energy industries comprise two separate catalytic, noncatalytic or physical unit processes. Here, in chemical processing, the first unit process is carried out in order to accomplish a desired chemical reaction and the second unit process serves for the regeneration of the inert or catalytic particulate matter used in the process. In this context, the term regeneration may simply refer to reheating of the solids, or in the case the particulate matter is a catalyst, to the reactivation thereof by means of heating. Correspondingly, in physical processes the solids medium can be used for transfer of heat or desired products from one unit process to another. Frequently, the chemical and physical processes are linked each other; in catalytic cracking, for instance, a chemical reaction occurs both in the reactor and the regenerator, complemented with physical processes (transfer of heat and material between the units).
In fact. heat exchange from one gas stream to another is one of the most crucial tasks in the process and energy generation technologies. Today, two heat exchanger types are in general use, respectively called recuperative or regenerative heat exchangers depending on the operating principle.
In recuperative heat exchangers, heat energy is transferred through a nonpermeable wall separating the flows from each other. In the basic type of recuperator, heat energy is directly conducted via the wall from one flow of medium to another flow. A specific subgroup of recuperators comprises so-called intermediate circulation recuperators, in which a heat-transferring medium is circulated between two recuperative heat exchangers. Such heat exchangers are employed in, e.g., nuclear power plants in which it is necessary to assure that the high-activity flow cannot mix with the secondary circulation in accident situations.
Another exemplifying group of intermediate-circulation recuperators is formed by fluidized-bed boilers equipped with superheaters placed external to the combustion chamber; in these boilers the sand heated in the combustion chamber is cooled in a separate fluidized-bed superheater. An example of such heat exchangers is described, e.g., in U.S. Pat. No. 4,552,203. The chief limitations of recuperative heat exchangers are related to the erosion, corrosion and temperature endurance of the heat exchanger vessel wall materials. Today, no practical wall materials are available for conditions exhibiting high mechanical or chemical stresses. The highest allowable temperature in recuperators is often limited by the strength properties of the wall material. Moreover, recuperators are expensive and restricted in their control possibilities. Good controllability can, however, be achieved in intermediate-circulation recuperators.
In regenerative heat exchangers, thermal energy is transferred by way of allowing the heated heat-transferring medium to release energy into a colder flow under a direct contact therewith and then reheating the cooled heat-transferring medium again under a direct contact in a hotter flow. Regenerative heat exchangers are further divided into cyclically and continuously operating types on the basis of their operating principle.
In cyclically operating regenerators, the hotter and the cooler flow are cyclically passed via a single solid structure which thus alternatingly stores and releases thermal energy. The batch-heated rock stove of a sauna is without doubt the oldest application of the cyclically operated regenerator.
In continuously operating regenerators, the heat-storing medium is continually recirculated from one flow to another. The best-known type of continuously operating regenerator is the Ljungstrom regenerator in which a rotating heat exchanger disc of cellular structure transfers thermal energy from one material flow to another. This regenerator type has been modified for different applications such as, for example, the air-conditioning regenerator which additionally provides moisture transfer on surfaces coated with lithium chloride paste.
Besides the regenerator types of the abovedescribed kinds with a fixed-shape, contiguous heat-transferring element, regenerators based on granular heat transfer media are known in the art.
Several different types of regenerators are known having the granular heat transfer medium in the fixed-bed state, and the heat transfer medium is then mechanically recirculated between the layers of the bed.
German Pat. No. DE 3,225,838 employs a granulated heat transfer medium (e.g., porcelain pellets) for heat transfer between the gas flows. The granular bed material is fluidized, whereby the pellets remain clean and clogging of the heat exchanger is avoided. U.S. Pat. No. 4,307,773 discloses another type of process and apparatus in which a regenerator system based on bubbling fluidized bed layers is employed for heat recovery from the gases of a hot contaminated fluid stream.
Besides the above-described patents, different types of regenerators are known based on alternate heating/cooling of granular material in separate, parallel. bubbling fluidized bed layers. UK Pat. No. 2,118,702 discloses a regenerator based on downward dribbling fixed bed layers.
A central issue of regenerators based on a fixed heat transfer element and fixed layered zones of granular material is how to keep them clean. Also the prevention of flows from mixing with each other causes sealing problems in these regenerators. Furthermore, the temperature differentials formed into the heat transfer material impose mechanical stresses which limit the life of the heat transfer element or material. A drawback of the layered fixed bed regenerator is the channelling of flows in the fixed bed layers. Moreover, the fixed bed layers obviously develop inevitable temperature gradients in the direction of the flow and the temperature of a layer is difficult to control.
One of the most generally used processes based on a fluidized-bed reactor system running two separate unit processes is the FCC equipment, which is intended for catalytic cracking of hydrocarbons, comprising chiefly a riser tube (reactor) operated in the fast fluidization flow state, cyclone separators of the catalyst and reaction product operated in a diluted suspension phase and a large-volume regenerator operated in the fluidized-bed state. An example of FCC equipment is represented by the embodiment illustrated in U.S. patent publication 4,957,617.
Other applications utilizing catalytic fluidized-bed reactors are, e.g.:
catalytic reforming, PA1 preparation of phthalic acid anhydride or maleic acid anhydride, PA1 oxidative dimerization of methane, PA1 Fischer-Tropsch synthesis, PA1 dehydrogenation, PA1 chlorination and bromination of methane, ethane and similar alkanes, and PA1 conversion of methanol into olefins or gasoline. PA1 thermal cracking, PA1 catalyst regeneration, and PA1 gasification processes. PA1 Suitable physical processes are, e.g.: PA1 drying, PA1 heat exchange between two gases, and PA1 adsorption. PA1 1. The symmetrically concentric construction of the CS reactors in the apparatus minimizes the horizontal transfer distances of the heat transfer medium also in large equipment.
Noncatalytic processes using fluidized-bed reactors are, e.g.:
In fluidized bed reactors, the flow velocities must be adapted according to the physical properties of the heat transfer material employed, and the control range of the regenerator is limited between the minimum fluidization velocity and the pneumatic transportation velocity. In practice this means that the heat transfer medium of the regenerator must have a coarse granular size, or alternatively, the flow velocities employed must be kept low. Furthermore. the recirculation of the heat transfer medium between the fluidized bed layers in a manner avoiding excessive mixing of the separated flows is problematic. This problem is accentuated at high pressure differentials between the heat-transferring flows. Herein, it is generally necessary to use mechanical valves whose wear and temperature limitations eliminate an essential portion of the benefits of this regenerator type. Prior-art fluidized-bed and fixed-bed regenerators require the use of a mechanical or pneumatic transfer arrangement for recycling the heat transfer medium from the lower unit to the upper unit. In terms of equipment and process technology, such transfer arrangements are almost impossible to implement.
Essential improvements to the above-described shortcomings are provided by the embodiment described in FI Pat. No. 924,438, in which the equipment comprises two or a greater number of parallel connected circulating fluidized bed reactors later in the text called the "CS" reactor. Of chemical processes, the catalytic cracking or dehydrogenation process among others can be constructed on an equipment configuration disclosed in the patent. However, the technical implementation of these apparatuses involves certain problems to be described in more detail below that prevent full utilization of these reactor apparatuses unless their limitations can be overcome. One of the most difficult problems herein relates to the long horizontal transfer distances of the circulating solids between the CS reactors that compel the constructions of large equipment to have a clumsy height.
Accordingly, if a number of CS reactors are arranged adjacent to each other, it is practically impossible to achieve a stable circulation of the solids without making the CS reactors inconveniently high. Also the design of communicating channels for the heat transfer medium poses construction problems. Further, the footprint required by adjacently located CS reactors will become intolerably large.